Chevreul, Michel-Eugène

Definition

Michel-Eugène Chevreul (1786–1889) is one of the most important chemists of nineteenth-century France. A pioneer of organic chemistry, he was twice President of the French Academy of Sciences. His work changed dramatically after his appointment as director of the dyeing department of the Gobelins Manufacture in Paris, where he worked for almost 60 years. At the Gobelins, he developed a considerable amount of work on color, including color classification, color applied to industry, as well as his most famous book on simultaneous contrast of colors, which had a great impact on several generations of artists as well as on color teaching. His exceptional longevity helped him to publish many books and hundreds of scientific papers, most of them on color topics. His 100th birthday was celebrated as a national event; he finally died at 103. His book The Principles of Harmony and Contrast of Colors and their Applications to the Arts [1] was once considered one of the 12 most important books on color [2].

Chevreul’s Life and Work

Born in Angers in 1786, Michel-Eugène Chevreul (Fig. 1) came to Paris when he was 17 years old and was appointed at the National Museum of Natural History as an assistant in charge of the chemical analysis of samples, thanks to a letter of recommendation from Vauquelin. Interestingly, his career as a chemist was determined by a sample of soap Vauquelin asked him to analyze. Indeed, the nature of animal fat was still unknown at the time. After several years of research, he published a book that gave him his fame as a chemist [3]. His discovery of the different acids contained in animal fat eventually led to important improvements in the field of industry, in particular in candles, as it made it possible to make candles shedding more light and less smoke. As he was also very interested in epistemological issues, he dedicated another book, one year later, to explain which method enabled him to make his discoveries [4].

Michel-Eugène Chevreul (1786–1889) at the approximate age of 50 (lithograph by Nicolas-Eustache Maurin, 1836, engraving by Conrad Cook)

In 1824, thanks to his fame as a chemist, Chevreul was appointed at the Gobelins Manufactures, as Director of the Dyeing Department. The Gobelins usually appointed chemists, as one of the main tasks of the Department of Dyes was to take care of the dyes of wools and silks used by the three manufactures, the most important being that of tapestries (Gobelins). The Director of the Department had indeed among other tasks that of looking after the quality of the wool (that had to be cleaned from grease and bleached) and the quality of the dyes according to their stability, their brilliance, and the kind of cloth to which they had to be applied (basically wool, silk, and cotton). Another important issue was that of color classification.

Before focusing this essay on the law of simultaneous contrast of color, it is worth noting the wide range of interests Chevreul had for colors. He himself suggested a classification of his work on color. The two main categories are (a) means of naming and defining colors and (b) dyes ([5], p. 121).

Means of Naming and Defining Colors

After being appointed at the Gobelins, Chevreul quickly realized that when the weavers needed a nuance of color, they used to show a sample of thread for matching, which was very empirical. For this reason, Chevreul felt it necessary to create a general classification of colors he first called “hemispheric construction” (Fig. 2), which, interestingly, is a black-and-white model (published in [1]). The circle is divided into 72 hues. Each of the 72 radii is divided into 20 segments, numbered from 1 to 20, corresponding to the scale of lightness, from the center (white) to the diameter (black). The third dimension is given by a quadrant perpendicular to the circle (unfolded in Fig. 2), corresponding to a saturation scale, and divided in 10 sections. This abstract system of color classification is the most complex realized at the time (1839) and permitted differentiating a great number of nuances: the 72 main hues of the circle with their 20 grades of lightness already give 1440 different nuances, to which must be added the 10 grades of saturation on the axe of the quadrant, i.e. 1440 × 9 = 12960. So the general amount of nuances is 14400 + 20 grey along the 10th radius, that is 14440. For a more precise account of Chevreul’s color classification and a reply to the critiques made to it – in particular the fact that he would have confused lightness and saturation – see [6, 7], pp. 163–172.

Aware of the importance of color classification beyond the case of the Gobelins, Chevreul went on working on the topic and published several important books containing, unlike the 1839 black-and-white hemispheric construction, beautiful color plates [8, 9, 10]. At the 1851 World Exposition in London, Chevreul’s chromatic circle (Fig. 3) was awarded a Great Medal.

Dyes

In his classification of his own work on color, Chevreul divided the dye section into three parts: all that is relative to the simultaneous contrast of colors, all that concerns what he called the principle of color mixing (which corresponds to what is known today as chromatic assimilation), and finally chemical researches.

Indeed, long before being appointed by the Gobelins, Chevreul had worked on natural tints; on indigo, for instance, he devoted a dozen papers, the first being published in 1807, when he was 20 years old [11]. His interest for animal fat also helped him to work on the process of degreasing and of bleaching dyes, to which he devoted numerous papers.

Although Chevreul’s work on color covers many aspects, his most important contribution to color is his law of simultaneous contrast of colors, as expounded in his book translated into English under the title The Principles of Harmony and Contrast of Colours and their Application to the Arts (1st edition in French, 1839; 1st English translation, 1854). Its starting point was a complaint from the weavers of the Gobelins against the dyers of the Department of Dyeing that he directed. The complaint was in particular about the black samples of wool used for the shades of blue and violet draperies. As a chemist, Chevreul first tested the wools dyed in black in his workshop and compared them with those dyed in the best places from London and Vienna. After a careful comparison, he realized that the quality of the dyed material was not in question. This led him to raise a brilliant hypothesis: the lack of strength of the blacks was not due to the dyes or their uptake but was a visual phenomenon related to the colors juxtaposed to the blacks. This new discovery was all the more surprising as Chevreul, being a chemist, was not prepared to admit that the cause of the phenomenon he observed “is certainly at the same time physiological and psychical” ([12], p. 101).

Indeed, Chevreul realized that it is not the same to look at a sample of color when isolated and when juxtaposed to another contiguous one. In the latter case, the two samples appear different from when seen in isolation. This is the most general effect of the law of simultaneous contrast of colors that reads, “In the case where the eye sees at the same time two contiguous colours, they will appear as dissimilar as possible, both in their optical composition and in the strength of their colour” ([1], § 16). Chevreul made it very clear in one of the plates (Fig. 4): O and O’, as well as P and P’, have exactly the same degree of lightness; however, the perception of the samples differs when they are seen in isolation and juxtaposed to another sample of a different degree of lightness. The bottom of the same plate shows an effect known as “Chevreul’s illusion”: each stripe (except the two extremes) being lighter than the following (when seen from left to right), a double effect is produced, because the left half of each stripe will appear darker and the right half lighter, due to the influence at the edges of the preceding and following stripes. (For Chevreul’s illusion, see [13].)

Illustration of the contrast of lightness; redrawn detail from original figure in M.-E. Chevreul, De la loi du contraste simultané des couleurs…, 1839

Chevreul’s demonstration is valid for hues as well as for lightness. For what concerns hues, the main definition of the law of contrast reads,

If we look simultaneously upon two stripes of different tones of the same colour, or upon two stripes of the same tone of different colours placed side by side, if the stripes are not too wide, the eye perceives certain modifications which in the first place influence the intensity of colour, and in second, the optical composition of the two juxtaposed colours respectively. Now as these modifications make the stripes appear different from what they really are, I give to them the name of simultaneous contrast of colours; and I call contrast of tone the modification in intensity of colour, and contrast of colour that which affects the optical composition of each juxtaposed colour. ([1], § 8)

The principle is exactly the same as for lightness: the modification consists in an exaggeration of difference. Yet, in the case of hues, what means exaggeration of difference? Chevreul’s starting point is the concept of complementary colors, i.e., colors considered as the most opposed. According to the knowledge of the time, Chevreul considered as complementary the following pairs of colors:

So the modification perceived when seeing juxtaposed colors consists in perceiving each color as slightly tinted with the complementary color of the juxtaposed one. This is the clever way Chevreul had to understand and solve the problem raised by the weavers when complaining of the bad quality of the blacks dyed in the Dyeing Department of the Gobelins. When seen in isolation, the blacks are perfectly black, but when seen juxtaposed to violet, they are slightly tinted with the complementary color of violet, that is, yellow, and will look accordingly yellowish. In order to solve the problem, Chevreul suggested the weavers to mix a few threads of violet with the blacks, so that they neutralize the effect of yellow and make accordingly the blacks look blacker!

A particular case must be mentioned: what happens when the two juxtaposed hues are complementary, for example, red and green? According to the law of simultaneous contrast, the red will be slightly tinted by the complementary color of green, that is, red, and will be perceived as redder. Conversely, the green will be slightly tinted by the complementary color of red, that is, green, and will be perceived accordingly as greener. In this case, the two hues are not modified in the sense of a transformation of the hue itself but enhanced.

Regarding the importance of Chevreul’s law of color contrast, some authors hold that it was not original since other scientists before him, like Prieur, had already discovered the law of color contrast ([14], p. 306; [15], p. 140). It is true that Prieur and others had already discovered similar phenomena. However, it might be recalled that Chevreul fairly acknowledged what he borrowed from other authors (including Prieur) since he devoted a chapter to the issue of the relationship between his experiments and those made by others before him ([1], § 120–131). For him, indeed, his main contribution is not the “discovery” of the color contrast but the fact of classifying and structuring these phenomena well described by his predecessors but considered as belonging to one single class, when Chevreul proposed to carefully distinguish different kinds of contrast, so that the simultaneous contrast is just one of them. It is defined as follows:

In the simultaneous contrast of colours is included all the phenomena of modification which differently coloured objects appear to undergo in their physical composition and in the height of tone of their respective colours, when seen simultaneously. ([1], § 78)

Besides simultaneous contrast, Chevreul distinguishes successive contrast, which includes all the phenomena that are observed when the eyes, having looked at one or more colored objects for a certain length of time, perceive, upon turning them away, images of these objects offering the complementary color of that which is proper to each of them ([1], § 79). This distinction is very useful and helped to differentiate phenomena until then confused; it is still in use, even though simultaneous contrast is often related today to chromatic induction, while successive contrast is generally associated with chromatic adaptation; for this reason, the concept of afterimages is often used today instead of that of successive contrast.

Chevreul also distinguished a mixed contrast ([1], § 81), which combines simultaneous and successive contrast; it occurs, for instance, when, after having looked at one color for a certain length of time, another color is looked at. In this case, the resulting sensation is a combination of the second color and of the complementary of the first one. Finally, Chevreul also added later a fourth contrast, the rotary contrast obtained with colored spinning disks [16].

It is out of the scope of this essay to discuss the main critiques addressed to Chevreul in particular the fact that he would have confused mixture of lights and mixture of pigments or simultaneous contrast and assimilation (for a full account of these issues, see ([7], pp. 93–102)).

Chevreul’s Influence on Artists and Artisans

Another striking fact is the huge influence Chevreul had on generations of artists and artisans, even before the publication of his book on simultaneous contrast in 1839, thanks to the public lectures he gave and that were attended by painters, but also wallpaper fabricants and many other color practitioners. The range of his influence is indeed impressive, from tapestry to stained-glass restoration, shop signs to gardening. Many reasons explain the success of his book, soon printed out (the second French edition, published in 1889, as well as the third one, published in 1969, have also been quickly printed out). One is that by dedicating a copious volume to this matter, he gave wide public access to phenomena until then discussed only in specialized scientific journals. Another is that by meticulously studying the applications of his law to almost all the fields of art and crafts, he moved from pure science to applied science and addressed himself to almost all those who use color. One more reason is that by addressing the issue of how to match and harmonize juxtaposed colors, he provided artists and artisans with practical rules and harmony advices quite useful in the situation painters and tapestry-makers are constantly confronted with, that is, using juxtaposed colors. Finally, as he had a great prestige as a scientist, the color harmonies he proposed were avidly read and followed by artists anxious of matching their colors and enhancing them. Interestingly, unlike what is generally assumed ([17], p. 196), Chevreul was not a partisan of the harmony of color contrast and never recommended painters to juxtapose complementary colors on their canvases. The reason is that for him the effect of simultaneous contrast always occurs naturally so that if a painter tries to imitate what he sees, he will exaggerate the effect instead of rendering it accurately.

Even though Chevreul’s teachings gave rise to misunderstandings, he nevertheless had an enormous influence on painters, from the 1830s up to the beginning of abstract painting. If his influence on Delacroix remains controversial, it is important for impressionism and crucial for neoimpressionism and van Gogh. From the 1880s onward, his work was challenged by more up-to-date theories (Helmholtz, Rood); however, he still had an influence, in particular on Delaunay but also on color music, due to the usefulness of the rules of successive contrast he established. Even the most important books still used in color teaching (Itten and Albers) owe a lot to Chevreul. For a comprehensive account of Chevreul’s influence on artists, see [7].